Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium, such as a disc. Modem disc drives include a head disc assembly comprising one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a drive motor for rotation at a constant high speed. Disc drive components within the head disc assembly, such as the hub of the drive motor, an actuator assembly, and a voice coil motor, are mounted to a base plate. A top cover attaches to the base plate to internally seal the head disc assembly. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (“heads”) mounted to a radial actuator arm for movement of the heads relative to the discs. The read/write transducer, e.g. a magneto resistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment.
The voice coil motor assembly is part of the actuator assembly and operates to rotate the actuator arms and the attached read/write heads in an arcuate path over the respective disc surfaces. The voice coil motor assembly includes a coil and a magnetic circuit comprising one or more permanent magnet sets and magnetically permeable pole pieces. The coil is mounted on a rear portion of the actuator body opposite the actuator arms so as to be immersed in the magnetic field of the magnetic circuit. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about a pivot shaft so that the actuator arms and the attached heads move in an arc across the disc surfaces.
Typically, a magnetically permeable bottom pole is mounted to the base plate and a magnetically permeable top pole is either mounted to the inner surface of the top cover or is mounted to the bottom pole (via spacers or “standoffs”) in spaced relation to both the bottom pole and the top cover. At least one permanent magnet set is positioned between and attached to one of the two poles. A gap between the magnet set and the opposite pole provides space for the coil to move in response to the application of varying DC signals to the coil.
FIG. 1 shows a head disc assembly of a conventional disc drive 100 that uses the spacer method to position the top pole. The disc drive 100 includes a base plate 102 to which various components of the disc drive 100 are mounted. A top cover 104, shown partially cut away, cooperates with the base 102 to form an internal, sealed environment for the disc drive 100 in a conventional manner. The components include a drive motor 106 that rotates one or more discs 108 at a constant high speed. Information is written to and read from tracks on the discs 108 through the use of an actuator assembly 110, which rotates during a seek operation about a bearing shaft assembly 112 positioned adjacent the discs 108. The actuator assembly 110 includes a plurality of actuator arms 114 which extend towards the discs 108, with one or more flexures 116 extending from each of the actuator arms 114. Mounted at the distal end of each of the flexures 116 is a head 118 which includes an air bearing slider enabling the head 118 to fly in close proximity above the corresponding surface of the associated disc 108.
During a seek operation, the track position of the heads 118 is controlled through the use of a voice coil motor (VCM) assembly 120, which typically includes a coil 122 attached to the actuator arm 114 on the opposite side of the bearing shaft assembly 112, a top pole 124, a magnet 126, and a bottom pole 128. The magnet 126 either defines a pair of magnets with opposite polarity lying in a common plane, or a single part with a transition zone between two faces of opposite polarity, so that the magnet establishes a magnetic field in which the coil 122 is immersed. The top pole 124 is attached in spaced relation to the bottom pole 128 with magnetically permeable standoffs or side posts 130. The controlled application of current to the coil 122 causes magnetic interaction between the permanent magnet sets 126 and the coil 122 so that the coil 122 moves in accordance with the well known Lorentz relationship. The top pole 124 and the bottom pole 128 provide a return path for the magnetic field passing through the coil 122. Furthermore, the standoffs or side posts 130 typically act together with the top and bottom poles 124 and 128 to form a closed magnetic field loop for the magnetic flux lines emanating from the magnet set 126. As the coil 122 moves, the actuator assembly 110 pivots about the bearing shaft assembly 112, and the heads 118 are caused to move across the surfaces of the discs 108.
FIG. 2 shows a generalized sectional view of the conventional voice coil motor 120 shown in FIG. 1. The bottom pole 128 is mounted to the base plate 102 by any conventional method, such as screws or an adhesive. The top pole 124 is mounted to the bottom pole 128, and thus to the base plate 102 via standoffs 130 such that the top pole 124 is spaced apart from the bottom pole 128. The top pole 124 and the top cover 104 typically form an air gap 123 therebetween. The permanent magnet set 126 is attached to the top pole 124 opposite the top cover 104. The coil 122 is attached to the actuator assembly (not shown) and positioned within an air gap 125 between the magnet set 126 and the bottom pole 128.
One of the problems with this conventional design is that the overall height of the disc drive 100 is fixed due to form factor limitations, and thus a portion of this limited space or “height” is wasted due to the inclusion of the air gap 123. A second problem with this conventional design is that it requires extra parts, such as standoffs 130, to both mount and properly position the top pole 124 within the head disc assembly. While the standoffs 130 help to maximize the magnetic field in the vicinity of the coil 122 by creating a closed magnetic path, it has been determined that the standoffs 130 tends to increase inductance within the windings of the coil 122. Increased inductance results in an increased resistance to a change in the current within the windings of the coil 122 and thus slower “seek” times for the voice coil motor 120. Specifically, the “seek” time of a disc drive 100 is the amount of time necessary to pivot the actuator assembly 110 so that the heads 118 move between different tracks on the discs 108. Because a quick change in current through the coil 122 is necessary for fast seek times, it is axiomatic that an increase in inductance within the coil results in slower seek times. In addition to adding to the complexity and cost of manufacturing a disc drive, a further problem with the standoffs 130 is that engineering tolerances in the manufacture of the standoffs can lead to variations in the magnetic fields produced by different voice coil motors 120
As noted above, one solution to the problem of the air gap 123 and the manufacturing complexity and expense of the standoffs 130 is to mount the top pole 124 directly to an inner surface of the top cover 104, such as by an adhesive or by welding the top pole 124 to the metallic cover 104. However, adhesives are expensive and may cause outgassing that can corrupt normal disc drive operation, while welding creates the potential for gaps to form between the two parts that may allow contaminants to be trapped and possibly escape to the interior of the head disc assembly. In any event, the process of attaching the top pole 124 to the top cover 104 requires extra manufacturing steps that increases the build time and thus the cost of a disc drive 100. A further difficulty with attaching the top pole 124 directly to the cover 104 is that modem top covers 104 are frequently formed with numerous recessed regions, cutouts and other features required to accommodate the different components within the drive 100. As a result, it is difficult to provide a smooth mounting surface on the inner surface of the top cover 104 for attaching to the top pole 124. Indeed, while a metal top cover 104 provides needed support for certain disc drive components such as the bearing shaft assembly 112, it is expensive to mold or stamp aluminum top covers 104 with the intricate shapes required by current disc drive designs.
Accordingly there is a need for a voice coil motor assembly that minimizes wasted space within the disc drive while reducing manufacturing costs by eliminating unnecessary parts and simplifying manufacturing steps. The present invention provides a solution to these and other problems, and offers other advantages over the prior art.